The invention relates to the field of radio-ecological monitoring and may be used to measure the content of radionuclides in different components of the environment, during the processing of results of measurement in a hardware-software complex, making it possible to deal with large masses of radio-ecological information. Furthermore, the invention may be used in the accumulation, storage, renewal and transmission of radio-ecological data with subsequent mathematical processing with the use of computer display of the results on an electronic cartographic base. More concretely, the claimed method may be used to determine the activity of beta/alpha radiating radionuclides, such as H-3, C-14, Co-60, Ni-63, Sr-90, Cs-137, U-236, Pu-239, Am-241 and others, in prepared liquid samples by processing apparatus spectra with the use of a liquid scintillation counter.
A method for measuring the activity of samples comprising several radionuclides with the use of a liquid scintillation counter is known (see U.S. Pat. No. 4,918,310, IPC G 01 T 1/204). For the presumed N radionuclides at least N+1 windows of a pulse height analyzer are selected and the number of pulses in each window is counted. Then for the presumed quench level, earlier obtained efficiencies of registration of separate radionuclides are selected for each window and N+1 equations are made, from which the true values of the activities of separate radionuclides are determined by the method of least squares. Then another quench level is selected, and the values of activity are calculated for it. This cycle is repeated until the minimum deviation of the sum of these activities from the measured activity is reached.
A method is known in which the spectrum of the sample being assayed is measured by means of a multichannel amplitude analyzer which is connected to an analog-to-digital converter. Its quench level is determined, the normalized spectra of separate radionuclides are calculated for this quench level, and, using the method of least squares, it is determined by which multipliers should the spectra of separate samples be multiplied in order to obtain a sum spectrum that is closest to the one being assayed, wherein the aforesaid multipliers are proportional to the true content of the radionuclides in the sample (see U.S. Pat. No. 5,134,294, IPC G 01 T 1/204).
The method which is most similar to the claimed method in respect to technical essence and achieved effect is the method for identifying radionuclides in a liquid scintillation sample (see PCT application No. PCT/FI90/00016, IPC G 01 T 1/204), in which the spectrum of the sample being assayed is measured, after which for a corresponding quench level, normalized model spectra of separate radionuclides are determined from a library of base spectra of separate radionuclides for different quench levels by the method of interpolation and extrapolation. Then the method of least squares is used to minimize the difference between the spectrum of a sample Pi and the sum of model spectra of separate radionuclides Mij, multiplied by the coefficients cj determining the activity of separate radionuclides. The expression being minimized is as follows:                     F        =                                            ∑              i                        ⁢                                          (                                                      P                    i                                    -                                                            ∑                      j                                        ⁢                                                                  c                        j                                            ⁢                                              M                        ij                                                                                            )                            2                                =          min                                    (        1        )            
where i is the No. of the analyzer channel, j is the index of a radionuclide.
However, the known methods do not make it possible to identify radionuclides that have low activity when there are radionuclides with high activity present in the sample. This is due to the fact that because of the static character of the value of Pi, the square of deviation of each addend in the sum (1) has a static spread of values of about xcx9cPi. Accordingly, during minimization, only the addend with a large value of the analyzer count of Pi will affect the expression F being minimized, while the addend with the small value of Pi will be xe2x80x9cabsorbedxe2x80x9d because of the aforesaid spread. This results in that identification of the radionuclides is carried out almost only according to the xe2x80x9cpeaksxe2x80x9d of the spectra, without taking into account the additional information which may be provided by the weakly active sections.
Furthermore, the known methods do not make it possible to identify alpha-active radionuclides on a liquid scintillation counter. This is due to the fact that the alpha spectra of different radioisotopes when measured on a liquid scintillation counter have spectra which are very close in shape and almost completely overlap. The spectrum of a sample, which consists of several alpha-active nuclides, in shape hardly differs at all from the spectrum of a separate radionuclide, in view of which existing methods are not capable of distinguishing the contributions of different alpha-emitters. This results in that existing methods are only used for monitoring the presence of alpha-active radionuclides, without determining the radioisotope makeup.
The object of the invention is to create a method for identifying radionuclides in samples, which ensures enhancement of the sensitivity of liquid scintillation analysis to radionuclides having low activity, where there are radionuclides having high activity in the samples.
Furthermore, an object of the invention is to create a method for identifying radionuclides in samples, which ensures an increase in the reliability of measurement and the possibility for carrying out liquid scintillation analysis of alpha-emitting radionuclides in the case where there are interfering radionuclides in the samples.
The indicated technical result is achieved in a method for identifying radionuclides in samples with the use of a liquid scintillation counter, the method comprising selecting a sample of the environment or a technological sample, preparing the sample for measurement on a liquid scintillation counter, measuring and recording the spectrum of the sample, grouping the measured spectrum of the sample, creating a model spectrum on the basis of a library of base spectra of radionuclides and determining the content of the radionuclides in the sample by minimizing the deviation of the model spectrum from the measured spectrum of the sample, in that in accordance with the invention, determination of the content of radionuclides in the sample is carried out, minimizing the deviation of the model spectrum from the measured spectrum of the sample according to the expression                     ∑        i            ⁢                        (                                                    P                i                            -                              M                i                                                                    min                ⁡                                  (                                                            P                      i                                        ,                                          M                      i                                                        )                                            +              δ                                )                2              ->    min    ,
where Pi is the measured spectrum of the sample,
xcex4 is a coefficient, on the value of which the stability of the minimization process depends (xcex4 greater than 0),       M    i    =            ∑      j        ⁢                  c        j            ⁢              M        ij            
is a model spectrum,
i is the number of a group,
j is the index of a radionuclide,
Mij are normalized spectra of separate radionuclides from the library of base spectra,
cj designates relative contributions of base spectra of radionuclides to a model spectrum, by varying the values of these contributions cj, after which the obtained values of relative contributions of base spectra of radionuclides cj are recalculated into values of absolute activities (Aj) of radionuclides in a sample according to the equation:
Aj=cj*P/Ej,
where   P  =            ∑      i        ⁢          P      i      
is the integral count of the sample,
Ej is the efficiency of registration for the j isotope.
Wherein when the measured spectrum of the sample is grouped, the boundary values of the groups Ni are preferably determined in accordance with the recurrent relationship:
Ni+1=Ni+[(i+1)/m],
where
i=1, 2, . . . , n,
n is the number of groups,
m is an integer depending on the number of channels of the amplitude analyzer of the liquid scintillation counter and on the necessary number of groups,
[(i+1)/m] is a sign for an integer part of the expression.
Wherein, it is preferable that minimization of the deviation of a model spectrum from the measured spectrum of a sample be carried out in several steps. Wherein, in the initial step a value of xcex4 not less than max (Pi, Mi) for any i, and zero values of relative contributions of base spectra of radionuclides to the model spectrum (cj) are selected. Varying the value of cj, the minimum is found, and at each following step xcex4 is reduced, repeating the process of minimization, wherein the results of calculation obtained at the preceding step are the initial meanings of the contributions cj for each following step of minimization.
Furthermore, it is preferable that determination of the content of radionuclides in a sample with use of step-by-step minimization of the deviation of a model spectrum from the measured spectrum of a sample be terminated when the process of minimization becomes unstable, i.e. when small changes in the meanings of relative contributions of base spectra of radionuclides in the model spectrum cj will result in a substantial change in the expression being minimized.
It is also preferable that in the case of a solid sample, when it is being prepared for measurement on a liquid scintillation counter, in order to reduce the effects of luminescence and quenching, annealing of the sample is carried out, acidic breaking up is carried out with the aid of concentrated nitric acid and hydrogen peroxide, the sample is concentrated by evaporation to a state of moist salts, the moist residue is converted into a hydrochloride solution by adding a hot solution of hydrochloric acid and is concentrated by evaporation with the periodic addition, at least twice, of distilled water, then, after cooling to room temperature, concentrated orthophosphoric acid is added to the obtained concentrated solution, it is concentrated by evaporation with the periodic addition, at least twice, of concentrated hydrogen peroxide and hot distilled water to the formation of a colorless viscous solution which is a mixture of pyrophosphate complexes of cations of the main components of the solid sample, the solution is cooled to room temperature, a saturated solution of a bivalent lead chloride is added thereto and carried into a scintillation cocktail, adding an emulsifier, and the mixture is thoroughly mixed, obtaining a countable sample prepared for spectrometric analysis.